1. Field of the Invention
The invention relates to a manufacturing method for a secondary battery.
2. Description of Related Art
Currently, secondary batteries which can be recharged and repeatedly used are widely used. A process of manufacturing such secondary batteries includes an inspection process for detecting a defective battery (for example, Japanese Patent Application Publication No. 2002-352864 (JP 2002-352864 A)).
In general, a secondary battery has, as a base element, an electrode group including a positive electrode, a negative electrode, and a separator. In the electrode group, the positive electrode and the negative electrode are completely separated from each other by the separator so as not to be in electrical contact with each other. However, in some cases, foreign matter is mixed in the electrode group due to various reasons in the secondary battery manufacturing process, and the positive electrode and the negative electrode come in contact with each other via the foreign matter. Such a phenomenon is called a small short circuit and causes various performance degradations such as a reduction in capacity.
When foreign matter is mixed in the electrode group, a reduction in performance may occur even when the reduction in performance is not exhibited at the beginning of use. The reason for an occurrence of such an event can be thought of as follows. For example, when foreign matter is present on the electrode, a part in which the distance between the electrodes is small is locally generated due to the foreign matter. In such a part, a current is easily collected upon charging and discharging, and thus when charging and discharging are repeated, a short circuit path is gradually formed and reaches a small short circuit. That is, it can be said that a secondary battery having foreign matter mixed therein includes a small short circuit.
Accordingly, the secondary battery having foreign matter mixed therein should be eliminated as a defective product. However, in many cases, even when foreign matter is mixed in the electrode group, a short circuit which affects battery characteristics is not reached immediately after the completion of the secondary battery. Accordingly, the short circuit is not easily detected by an initial capacity inspection or voltage inspection.
Various methods have been proposed to detect the small short circuit. For example, JP 2002 352864 A discloses a method of detecting a small short circuit by performing charging and discharging of a secondary battery under a predetermined condition and by then measuring a change in battery voltage during leaving.